Transcript SEARCH FOR AXIONS AT CERN K. Zioutas Univ. Thessaloniki & Patras/Greece SPSC Villars meeting, 22-28/9/2004 27th September 2004

SEARCH FOR AXIONS AT CERN
K. Zioutas
Univ. Thessaloniki & Patras/Greece
SPSC Villars meeting, 22-28/9/2004
27th September 2004
Contents
CAST  – 2004 
 1st phase
 maxion< .03 eV/c2 +upgrade
 PRL to be submitted
CAST  2005 – 2007  2nd phase
 maxion < .8 eV/c2
……………………………………………………………………………
CAST  2008 –
 H2-anticryostat  maxion < 1.5 eV/c2
……………………………………………………………………………
Beyond CAST-baseline:
Axion(-like) particles:
• Light
 PVLAS-claim
 test PVLAS @ CERN ?
• Massive
 Kaluza-Klein axions
 direct detection
 with CAST
 with a large TPC
 indirect signature  solar X-rays
 1’
CAST X-ray Alignment
X-ray spot on CCD. Vertical = +8o
2004  stable
Improvements: solar core temperature gradient
Y. Semertzidis / TU-Darmstadt
Solar axion spectra = ƒ(r/Rsun)
Kaluza-Klein (KK) axions  Horvart, Krcmar, Lakic PRD 69 (2004) 125011
Search for solar nuclear M1-transitions
 The Sun is the strongest source of M1 transitions, e.g.
14.4 keV
2.2 MeV
5.5 MeV
 High Energy Calorimeter
Motivation:
• Broad band axion search with the high axion-to-photon
conversion efficiency inside the CAST magnetic pipes.
• Axion coupling to nuclear magnetic dipole (M1) transitions.
First measurements with the CAST H.E. Calorimeter.
 detector: CdWO4 crystal 0.6 kg (Ø45mm x 50 mm)
Comparison of energy spectra acquired during solar tracking (9.28 h) and normalized
background measurements (130.5 h) with very moderate software cuts. Counting rate
over full energy spectrum above 200 keV after cuts ~1.65 Hz. Correction to local background conditions in different magnet positions is not included yet.
CAST 2nd phase with buffer gas


2005 / 2006  below ma ~ 0.35 eV
3He
2006, 2007  below ma ~ 0.8 eV
……………………………………………..
 H2 / anticryostat
beyond 2007  below ma ~ 1.5 eV
4He
Improvements:
• Upgrade of the detectors & shielding
• Additional more efficient X-ray telescopes
• Accurate solar tracking  utilize solar temperature gradient
MOTIVATION:
• crossing of the theoretical line (gaγγ  ma ) with the best astrophysical
limit (see exclusion plot), while the cosmologically allowed axion rest
mass region from evaluated WMAP data is below 2-3 eV/c2.
• search for massive axions (also of the Kaluza-Klein type).
CAST’s 2nd PHASE: relevant relations
Extend coherence for a  g transitions to higher ma values.
Fill the magnetic pipes with Helium gas
4 N e
Z
mg 
 28.9
 eV
me
A

|q|=
|ma2 – mg2|
2E
mg > 0.
Ne: electron density
 : gas density (g/cm3)
(qL <<1 for coherence)
Max. density ~ 0.3x10-3 g/cm3 limited by 4He saturated vapour
pressure at 1.8°K
mg ~ 0.35 eV.
To reach higher mg values we need 3He as buffer gas.
Plots without absorption
4.2 keV
All energies
Cryowindows for 2nd phase of CAST
Tapio Niniikoski
Two Ø8 mm windows (Metorex) tested at the cryolab of CERN.
 leak rate at 4 K < 3·10-9 mbar·litre/s
 compatible with the requirements of 2nd phase of CAST.
 see Cryolab Note 03-04
this rate is likely due to back diffusion of helium from the atmosphere of the lab, or due to
contamination of the vacuum system by helium. The lack of any correlation between the helium
pressure and leak signal enables to conclude that the window leak rate was «10-9 mbar·litre/s.
The leak rate at room temperature had a similar upper limit and pressure variation. The two
windows tested showed identical behaviour, but one was prematurely broken because of the
Taconis effect, which made the window fail. This effect was avoided in the subsequent tests.
The yield of the hermetic 8 mm windows is less than 0.9 basing on rejection rate due to leaks.
Depending on the character of the leaks, the yield may then become less than 1.6% for perfectly
hermetic 50 mm windows, and we may have to accept a diffusion leak, which vanishes at low
temperatures. Pin holes with visible leak rate at RT cannot be accepted for LT use. The yield
becomes a problem in this case.
In addition:
Design study in preparation  T. Niniikoski & N.A. Elias
Ongoing simulation
 CAST
 At present, the plan for
the 2nd phase of CAST seems feasible.
 While preparing for VILLARS & looking to the future
of CAST, we identified 
(Alternative) solution (~2007 - ):
T.Niniikoski
1)
4He/
H2 inside anticryostat  ~ 2x axion rest mass
A warmer gas cell solution (5.5K for 4He or 30K for H2) avoids the window
problems. It also avoids the need for high stability of the temperature and pressure,
because the filling is almost constant for a closed system. The physics potential is
better, because of the higher axion rest mass reach.
The cost of the system is likely to be much lower than that of the 3He.
 design study  2005
2) Si-diodes as cryo-windows & X-ray detectors 2007Cold silicon detectors have been operated successfully in S134 first time in 1974, with
Ø1.5cm. They would be compatible with CAST ~1 keV threshold requirements, if a
modern low-noise preamplifier with long integration time would be used.
Background, noise & threshold measurements can be made by RD-39 at short notice.
 necessary for ma > 0.8 eV/c2
Sofar:
this is the maximum CAST performance
we think we can achieve with X-rays.
 PVLAS claim !
PVLAS-experiment
M1 & M2
 very high reflectivity
dielectric mirrors
 Fabry-Perot optical
resonator
 1 msec
LASER
 linearly polarized light
 elliptical polarized
Magnet: http://www.ts.infn.it/experiments/pvlas/magnet/pict-magnet/cryogen.jpg
x
NOTE: KK- axions

PQ-axions
Test PVLAS @ CERN
“Light shining through a wall experiment”
`
 Possible options:
•
•
•
•
CAST + 1 LHC magnet
CAST/2
2 LHC magnets @ SM18
lowest energy solar axions  CAST
Light shining through a wall
An experiment to verify the interpretation
of the PVLAS results as an axion effect.
L. DiLella
The PVLAS Collaboration has recently measured abnormally large elipticity and
polarization rotation of laser light undergoing multiple reflections in a dipole magnet,
being consistent with the effects expected from an axion with a mass ma10-3 eV/c2
and a coupling constant gaγγ = (2-3)·10-6 GeV-1. If these results are confirmed, and
assuming -as a working hypothesis- that all other experiments that have already
excluded the PVLAS region of axion parameters are wrong for some reason, it is
important to verify the PVLAS results using an independent experiment. This could
be an experiment of the type called “Light shining through a wall” :
A laser beam traverses a first dipole magnet where photon-axion transitions occur.
The axion then traverses a wall and is converted back to the original photon in another
dipole magnet. Obviously, the amount of light shining through the wall is proportional
to (gaγγ)4, while the magnitude of the effects measured by PVLAS is proportional to
(g aγγ)2. Nevertheless, with the use of two decommissioned LHC magnets,it is possible to
reach a sensitivity to g aγγ values as low as 10-7 GeV-1 by multiple reflections of
the laser beam in the first magnet.
The rate of photons “shining through the wall”, Rγ, is given by
Rg  ( Pag ) 2
W n

Eg 2
(1)
where W is the power of the laser beam, Eγ is the photon energy, n is the
number of reflections in the first magnet (only the photon paths pointing
to the wall are useful), and η is the photon detection efficiency.
The axion-to-photon conversion probability is given by:
2
Pag
 g agg
 sin 2 (qL / 2)
 
BL 
2
 2
 ( qL / 2)
(2)
where gaγγ is in GeV-1, B is the magnetic field (in Tesla), L is the
magnet effective length (in metres) and q is the momentum transfer to the
magnet. For gaγγ=10-7 GeV-1, B = 9 T, L = 9.26 m, and assuming that
q = 0 (see below), Paγ  1.7·10-11.
We use a green laser (λ= 514.5 nm, Eγ = 2.41 eV) with an average power
W = 2 mW. The value n = 2·105 can be obtained using commercially
available mirrors to create a Fabri-Perot resonance cavity in the space
between them (this requires that the distance between the two mirrors
be adjusted to correspond to a multiple of the wave-length λ). Finally, we
assume a photon detection efficiency η=0.5, which can be obtained using
Visible Light Photon Counters (VLPC).
Using these values, we find:
Rγ  0.08 / s
which corresponds to ~5 counts per minute above noise. Hence the VLPC
noise should be reduced to much less than this value, if possible. The laser
should be pulsed and the VLPC should be gated for ~0.01 s after the laser
pulse to allow for the multiple light reflections.This will reduce the VLPC
noise contribution to Rγ. The laser light should be linearly polarized and
data should be taken by alternating the polarization between a direction
parallel to the magnetic field and a direction orthogonal to it. No signal is
expected in the latter case, giving further evidence for an axion effect if
an excess of counts above noise is observed.
The momentum transfer to the magnet has been assumed so far to be zero.
In the presence of gas in the magnet gap, it is given by the expression:
ma - mg
q
2 Eg
2
2
where mγ is an effective photon mass. For optical photons:
v 1
 
c n
Eg - mg
2
Eg
2
1-
mg
2
2 Eg
2
(3)
where n is the refraction index. For gases n=1+αρ, where ρ is the density
in g cm-3. Using 4He (α = 0.1954) and setting mγ=10-3 eV/c2 in Eq. (3),
we find ρ = 4.4·10-7 g cm-3, which is well below the density corresponding
to the 4He saturated vapour pressure at 1.8 K (ρsat  3·10-4 g cm-3).
In vacuum (mγ = 0), for ma = 10-3 eV/c2 the momentum transfer to the
magnet is q = 2.07·10-7 eV/c, corresponding to a wavelength of ~6 m,
giving qL/2 = 4.86. Using this value in Eq. (2) squared reduces the
photon rate Rγ by about three orders of magnitude.
Since the axion mass is not precisely known, the 4He density should be
varied in small steps, or varied continuously during data taking. A
relative variation Δρ/ρ2·10-4 near ρ = 4.4·10-7 g cm-3 changes the axion
mass for which q = 0 by 10-4 eV/c2. Obviously, a scan of Rγ as a function
of the gas density is expected to show the oscillatory behaviour predicted
by Eq. (2), thus providing additional evidence for an axion effect.
……………………………………………………………………………………………………...
Alternative suggestions:
• M. Davenport
2x15 m LHC-magnets in SM18  ~ 2007 - ?
• R. Kotthaus
CAST magnet only
A high power UV-LASER (~10 W, λ=200 nm)
 without optical cavity & buffer gas
A wall at the center of the magnet
 Rate ~ a few Hz
CAST operation in the visible: continuous & discret lines
• visible photon  axion inside Bsolar near the photosphere
• axion coupling to forbidden atomic M1 transitions,
e.g. the green (Fe-XIV) and red (Fe-X) lines in solar atmosphere
 outer Sun = source of ~ eV axions ?
 Needed: CAST + single photon sensitive detectors in the visible
 film, APDs (IR to vacuum UV), PMTs  noise = ?
 first test run already in 2004 ?
 MOTIVATION:
a) lowest threshold solar axion search with CAST
b) test PVLAS with CAST ?
CAST performance in the visible
with PVLAS results & solar input
 PVLAS:
gaγγ ≈ 2.5·10-6 GeV-1 & maxion ≈ 10-3 eV/c2.
Above the solar photosphere, we take:
• Bsolar≈ 9 Gauss.
• solar oscillation length L ≈ 1 km.
 at the solar surface the density (ρ ~10-4 bar) is decreasing exponentially outwards. In order to have maxion≈mγ inside the solar atmosphere
(as for CAST 2nd phase), a ρ ≈ 10-5 bar is needed. Therefore, above
the solar surface the photon-to-axion conversion can be enhanced in
the axion rest mass range ~ 10-2 to ~10-5 eV/c2. I.e., for a distance of
~1 km the local density is the required one to restore coherence.
• Lsolar ≈ 4·1033 erg/s.
 Pγa ≈ 6·10-13
 Φ ≈ 106 axions / sec·CAST-exit
In CAST:
 Pa γ ≈ 10-9 (assuming ~5 m oscillation length)
 Rate = Pa γ · Φ ≈ 10-3 photons / sec·CAST-exit
Note:
this is probably a conservative estimate. The solar oscillation length
may be taken ~10 km, since the opacity in the visible seems to be
reasonable for some 1000 km above the photosphere. Also, the
local (quiet) Bsolar might be even larger with peaks at ~1.5 kGauss.
[see F. Cataaneo, ApJ. 515 (1999) L39; S.R. Cranmer,
astro-ph/0409260; R.M. Sainz et al., ApJL. 614 (10.10.2004)].
Thus, the photon rate during solar tracking with CAST can be
 Rate ~ 10-3  1 visible phot. / sec·CAST-exit
R. Schwenn et al.,
Sol. Phys. 175 (1997) 667.

 Green Fe-XIV M1 line
@ 530.3 nm
?
Green line only
Axion  atomic M1 transitions
Z., Semertzidis, PLA130 (1988) 94.
Rsolar
Note the different radial shapes.
Beyond CAST
 Direct search for solar massive axion(-like) particles, e.g. of the
Kaluza-Klein type with a large volume chamber:
 ALICE-TPC
  ‘trigger’ ? noise ?
 gaγγ 
 τ ~ (maxion)-3  “short lived” (~1020 s)
Signal due to spontaneous / B-induced decay of axions:
a) 2-prong events
 Eγ1≈Eγ2≈1-10 keV & Rate ~ 1/m3day
b) 1-prong events inside B  single X-ray photon below ~10 keV
 rate = ?
 Present indirect limit: ~ 20000 / m3day
preferred place: underground + shielding
Motivation:

 “a first”
Solar corona heating problem 1939-
… one of the longest unsolved mysteries in all of astrophysics
Schmelz,
Adv. Space Res. 32 (2003) 895
 Suggested solution within astroparticle physics:
• decay X-rays from accumulated solar massive
axions of the Kaluza-Klein type, gravitationally
trapped by the Sun over 4.6 Gyears.
Observational evidence for gravitational
trapped massive axion(-like) particles
DiLella, Z., Astropart. Phys. 19 (2003) 145
?
The solar X-ray spectrum reconstructed from the emission measure distribution (EM(T)) for the
non-flaring Sun at the solar minimum [16]. A thermal component of ~1.8 MK is also shown
(blue line). (EM(T) is approximately the product of the square of the electron density with the
emitting volume V(T) as a function of temperature). Red line : solar KK-axion model
[16] Peres, Orlando, Reale, Rosner, Hudson, ApJ. 528 (2000) 537
Quiet Sun X-rays as Signature for New Particles
Z., Dennerl, DiLella, Hoffmann, Jacoby, Papaevangelou
ApJ. 607 (2004) 575
X-ray activity is
connected to strong B
e.g. J. Qiu et al.,
ApJ. 612 (2004) 530
 solar axions  B ?
 search 1-prong events
YOHKOH: IAlMg (<4keV) ~ Bn dependence as a function of time.
“The relation between the soft X-ray flux … and … the magnetic flux
can be approximated by a power law with an averaged index close to 2.”
Benevolenskaya, Kosovichev, Lemen, Scherrer, Slater ApJ. 571 (2002) L181
 axion-to-photon conversion  B2 
Then : 1) radiative decay
2) interaction with BSOLAR
Hoffmann, Z. in preparation
 constant term
 also local effects
3) axion - condensate(s) ?

? 11-years solar cycle ?
Summary:
•
CAST proposal 9.8.1999
•
Improvements/extensions:
X-ray telescope
 ~ arcmin space resolution
Point to other celestial sources (parasitic runs)  background
High energy calorimeter
•
2003 data  5 times better limit for gaγγ  PRL paper
•
2004 data  upgraded performance  high quality data
•
•
2005-2007  2nd phase of CAST  maxion below ~ 0.8 eV/c2
2008 - with H2  maxion below ~ 1.5 eV/c2
•
PVLAS claim & spontaneous axion decays
(e.g. of the Kaluza-Klein type) were not in sight in 1999.
•
Motivation for further work: test PVLAS, search for massive
axion(-like) particles & theoretical/observational studies.
M.J. Aschwanden,
Physics of the Solar Corona (2004)